Optical imaging methods offer a number of advantages over other imaging methods. Such imaging typically uses light in the red and near-infrared (NIR) range (600-1200 nm) to maximize tissue penetration and minimize absorption from natural biological absorbers such as hemoglobin and water. Optical imaging may provide high sensitivity, does not require exposure of test subjects or laboratory personnel to ionizing radiation, can allow for simultaneous use of multiple, distinguishable probes (which may be important in molecular imaging), and offers high temporal and spatial resolution, which is important in functional imaging and in vivo microscopy, respectively.
In fluorescence imaging, filtered light or a laser with a defined bandwidth is used as a source of excitation light. The excitation light travels through body tissue, and when the excitation light encounters a reporter molecule (for example, a contrast agent or imaging probe), the light is absorbed. The reporter molecule then emits light that has detectably different properties from the excitation light. The resulting emitted light then can be used to construct an image. Most optical imaging techniques have relied on the use of organic and inorganic fluorescent dyes as the reporter molecule.
Fluorescent dyes are generally known and used for fluorescence labeling and detection of various biological and non-biological materials by procedures such as fluorescence microscopy, fluorescence immunoassay, and flow cytometry. A typical method for labeling such materials with fluorescent dyes is to create a fluorescent complex by means of bonding between suitable groups on the dye molecule and compatible groups on the material to be labeled. In this way, materials such as cells, tissues, amino acids, proteins, antibodies, drugs, hormones, nucleotides, nucleic acids, lipids and polysaccharides and the like may be chemically labeled and detected or quantified, or may be used as fluorescent probes which can bind specifically to target materials and detected by fluorescence detection methods. Brightly fluorescent dyes permit detection or localization of the attached materials with great sensitivity.
For many years, polymethine dyes have been useful as sensitizers in photography, especially in the red and near infrared regions of the spectrum. However, in more recent years, these dyes have been used in a variety of different innovative technological areas, such as laser and electro-optic applications, optical recording media, and medical diagnostic applications. Such applications place high demands on dyes, for example, on the degree of purity required and the reproducibility of synthetic methods. Polymethine dyes are useful as labeling agents in biological applications, especially when such dyes include functional groups that are capable of forming a stable covalent bond with a biological molecule of interest so that the resulting conjugate will function as an efficient fluorescent biological analyte or probe.
Notwithstanding, there is an ongoing need for new dyes that can be used in various medical, diagnostic and biological applications.